This invention relates to a circuit for the linear measurement of a current flowing through a load, of a type which comprises a current sensor, and a first driver transistor connected to the load and a second transistor connected to said current sensor, both said transistors being field effect transistors and interconnected into a current mirror configuration.
As is known, circuits of the above-noted type find application to a variety of devices for measuring and controlling a current flowing through a load, such as current regulators and limiters for a current supplied to an electric motor, which will be referred to hereinafter as resistive load R.sub.L.
A current flowing through the load R.sub.L is usually measured indirectly by measuring the voltage drop V.sub.S across a resistor R.sub.S connected to the load serially.
In this respect, a first known technical approach consists of driving the load current I.sub.L through a field effect driver transistor, such as a MOSFET, having its drain electrode connected to the load R.sub.L and its source electrode connected to ground through the resistor R.sub.S.
However, this first approach has the drawback that a power is dissipated across the resistor R.sub.S which is equal to the voltage drop V.sub.S multiplied by the load current I.sub.L.
The prior art proposes a second approach in order to obviate the drawback and restricting the power dissipation while ensuring a reliable measurement.
The second prior approach consists of providing a measuring circuit which comprises a pair of field effect transistors of either the MOSFET or its equivalent JFET types, connected to each other into a current mirror configuration. In particular, the first and second transistors which make up the above-noted pair have their respective drain electrodes connected to the load R.sub.L and their respective gate electrodes connected together. Further, whereas the first transistor is a driver transistor and has its source electrode connected directly to ground, the second transistor has its source electrode grounded through the current sensing resistor R.sub.S.
Furthermore, by making the second transistor with an area which is n times smaller than that of the first transistor, it would be possible to flow through the drain of the second transistor, and hence the resistor R.sub.S, a current I.sub.S which is n+1 times smaller than the load current I.sub.L. The power dissipated across the resistor R.sub.S would then be n+1 times smaller than that dissipated with the former known technical approach.
However, that possibility is hindered in that the presence of the resistor R.sub.S makes the values unequal of the gate-source and drain-source voltages of the transistors which are in different operating conditions. As a result, the serious problem is encountered that the value of the n parameter, indicating the current mirror ratio for the transistor pair, is made dependent on the value of the voltage drop V.sub.S tied, in turn, to the load current I.sub.L as well as to temperature, as related in the EDN scientific magazine issue of Sept. 4, 1986.
Thus, the measurement of the load current I.sub.L turns out to be non-linear and dependent on the operating temperature.